Make inactivity timeout configurable

This PR is for issue #1408 and makes the inactivity timeout configurable, so e.g. the controls can be hidden quicker or slower than the default 2000 ms:

    <script>
        videojs.options.inactivityTimeout = 5000;
    </script>


I have not dug into the code enough yet to know how to do that, but I'll take a look tomorrow.